Two piece oil control ring with nitrided surface layers

ABSTRACT

A two piece oil control ring is provided which comprises a ring body ( 1 ) formed into a generally M- or I-shaped section with an annular groove ( 4 ) on an inside surface ( 3 ) of the ring body ( 1 ); and a coiled expander ( 2 ) attached in the annular groove ( 4 ) of the ring body ( 1 ) for resiliently urging the ring body ( 1 ) toward an inner wall of a cylinder. The ring body ( 1 ) is made of stainless steel that contains chromium of 3.0 to 13.0 weight % with the outer and inner circumferential surfaces ( 11, 12 ) covered with nitrided layers ( 14, 15 ). Circular ratio (τ) of an outer edge length (Y o ) to an inner edge length (Y i ) in the cross section of the ring body ( 1 ) is in a range between 1.08 and 1.32 to keep precise peripheral circularity of the ring body so that the nitrided layers survive lapping of the ring body ( 14, 15 ).

This invention relates to an oil control ring attached in a ring grooveof a piston in an internal combustion engine, in particular to a twopiece oil control ring that has a coiled expander and a ring body withnitrided surface layers formed thereon.

BACKGROUND OF THE INVENTION

Various types of coil expander loaded oil control rings have generallybeen utilized in most of diesel engines. A typical example of two pieceoil control rings comprises, as shown in FIG. 6, a ring body 1 with anannular groove 4 on an inside surface 3 of ring body 1; and a coiledexpander 2 attached in annular groove 4 of ring body 1. Ring body 1comprises an upper rail 5, a lower rail 6 and a web 7 that connectsupper and lower rails 5, 6 to form a generally M-shaped section. Aplurality of openings 8 are formed in web 7 to allow lubricating oil topass through the openings 8.

Not shown but, wear resistant surface layers are formed on outerperipheral surfaces of upper and lower rails 5, 6, and on innerperipheral surfaces of ring body 1 with annular groove 4. Coiledexpander 2 can produce elastic force to resiliently urge each outerperipheral surface of upper and lower rails 5, 6 toward an inner wall ofa cylinder not shown. Cross sectional shape of ring body 1 is determinedin considering followability in operation of deformable ring body 1 toinner circumferential surface of cylinder, tangential force of oilcontrol ring, stress of ring body 1 produced when attached in a grooveformed in piston, running stress and workability of wire rods into ringsso that the oil control ring can perform such essential function as toapply an appropriate amount of lubricating oil on the inner surface ofcylinder. In response to recent requirement of higher power and lowerfuel consumption for diesel engines, ring bodies are formed of steel inlieu of cast iron, and wear resistant surface layers are formed bynitriding instead of hard chromium plating to improve wear andscuff-resistance of ring body.

Japanese Patent Disclosure No. 6-336951 shows an oil control ring thathas a ring body made of 17% chromium stainless steel with ion-nitridedlayers formed on outer and inner circumferential surfaces of ring body.In making such an oil control ring with ion-nitrided layer, a drawingprocess is used to produce a deformed wire rod of a given usual crosssection, and then a plurality of openings are formed in a web of thewire rod by punching. Then, the wire rod is formed into a coil and cutinto ring bodies, lapped with outer circumferential surfaces of the ringbodies, ground with upper and bottom surfaces of the ring bodies, andnitrided with whole circumferential surface or necessary areas of thering bodies. After nitriding, the ring bodies are ground in a similarway to finish oil rings of final shape.

Some attempts have recently been made to fabricate a two piece oilcontrol ring made of stainless steel of low chromium content less than17 weight % to reduce manufacturing cost. This oil control ring is,however, disadvantageous in that roundness or circularity of ring bodyis remarkably deteriorated with warpage upon nitriding expansion duringand after a nitriding process because the process involves unbalancedstresses produced in inner and outer nitrided layers of ring body formedinto M- or I-shape made of stainless steel having low chromium content.When nitrided layers are formed on whole circumferential surfaces of M-or I-shaped ring body 1, the nitrided layers are expanded to generatecompression stresses in the nitrided layers, and thereby to causetensile stresses in the base material and incidental distortion in theinner and outer circumferential sides of ring body 1. If distortionproduced in the outer circumferential side is greater than that producedin the inner circumferential side, the outer circumferential sidenaturally expands with longer extended length than that of the innercircumferential side, deforming end portions of ring body 1 toward theinner circumferential side. When the lapping process is applied tocircumferential surfaces of such deformed ring, the end portions of ringbody cannot uniformly be lapped due to distortion by nitriding. When anoil control ring of degraded circularity is attached to a piston, itprovides a clearance or gap between an inner surface of a cylinder andouter circumferential surface of the oil ring, thereby resulting indetrimentally increased oil consumption. Otherwise, when the whole outercircumferential surface is lapped to a required periphery circularity ofring body, the nitrided layer is disadvantageously removed by lapping,and therefore, after nitriding, lapping of the ring body is extremelyrestricted or should be performed very carefully. On another aspect, italso has been found that reduced chromium content deteriorates creepresistance of ring body.

FIG. 5 is a graph showing increase of lapping amount and elapse oflapping time in a proportional relationship. The term “lapping” referredherein means any process for polishing, grinding, scouring, scrubbing orabrading a ring body with lapping agent to adjust or compensatedimensional error or to improve surface finish during manufacture. Forexample, a nitrided layer of 70 micrometers thickness is thoroughlyremoved through a lapping process of 105 seconds. Accordingly, lappingtime should be within 105 seconds to surely leave nitrided layers onwhole outer circumferential surfaces of ring body 1 so as to bringlapped profile of nitrided layers in ring body 1 into contact to innersurface of cylinder. A prior art ring body of 17% Cr stainless steelproduces similar unbalanced stresses in inner and outer nitrided layers,but indicates very little amount of strain due to nitridation because17% Cr stainless steel has greater mechanical strength, and wholecircumferential surfaces of the ring body can be lapped by usual processwithout presenting any substantial problem. However, cost problem isstill involved in 17% Cr stainless steel.

An object of the present invention is to provide a two piece oil controlring that has a ring body capable of keeping such peripheral circularitythat nitrided layer formed on the ring body survives lapping. Anotherobject of the present invention is to provide a two piece oil controlring with a ring body that can control or restrict amount of deformationproduced when nitrided layer is formed on a surface of the ring body.Still another object of the instant invention is to provide a two pieceoil control ring with a ring body made of stainless steel having lowchromium content.

SUMMARY OF THE INVENTION

A two piece oil control ring according to the present invention,comprises a ring body (1) formed into a generally M- or I-shaped sectionwith an annular groove (4) on an inside surface (3) of the ring body(1); and a coiled expander (2) attached in the annular groove (4) of thering body (1) for resiliently urging the ring body (1) toward an innerwall of a cylinder. The ring body (1) comprises upper and lower rails(5, 6) and a web (7) that connects the upper and lower rails (5, 6), theweb (7) being provided with a plurality of openings (8) for allowinglubricating oil to pass therethrough. The ring body (1) is made ofstainless steel that contains chromium of 3.0 to 13.0 weight % with theouter and inner circumferential surfaces (11, 12) covered with nitridedlayers (14, 15). A circular ratio (τ) of an outer surface (edge) length(Y_(o)) to an inner surface (edge) length (Y_(i)) in the cross sectionof the ring body (1) is in a range between 1.08 and 1.32 to keep preciseperipheral circularity of the ring body (1) with less strain ordeformation by nitridation and thereby to allow the nitrided layers (14,15) to survive lapping of the ring body (1).

The above-mentioned objects and advantages of the present invention willbe apparent from the following description in connection with preferredembodiments shown in the accompanying drawings wherein:

FIG. 1 is a perspective sectional view of a ring body with nitridedlayers used in a two piece oil control ring according to the presentinvention;

FIG. 2 is a cross section of the ring body generally formed intoM-shape;

FIG. 3 is a cross section of the ring body generally formed intoI-shape;

FIG. 4 is a graph showing a relationship between lapping time and variedratio of outer to inner surface lengths;

FIG. 5 is a graph showing a proportional relationship between lappingamount and lapping time; and

FIG. 6 is a sectional view of a ring body used in a prior art two piecetype oil ring.

DISCLOSURE OF THE PRESENT INVENTION

FIG. 1 illustrates a ring boy 1 used in an oil control ring according tothe present invention. Ring body 1 is formed of stainless steelcontaining chromium of 3.0 to 13.0 weight %, and is coated with nitridedlayers 13 that comprises an outer nitrided layer 14 formed on an outercircumferential surface 11 of ring body 1; and an inner nitrided layer15 formed on an inner circumferential surface 12. Similarly to prior artoil control ring shown in FIG. 6, ring body 1 of FIG. 1 is formed with aplurality of openings 8 through which lubricating oil passes. Ring body1 has its width B: 1.2 to 2.0 mm, thickness t: 1.2 to 2.0 mm, land(rail) width l₀: 0.15 to 0.22 mm and annular groove depth t₀: 0.5 to0.75.

Nitridation expansion of ring body 1 generates different deformation ofouter and inner nitrided layers 14 and 15, resulting in warpage of ringbody 1, and it would be necessary to balance compressive forces resultedfrom nitridation expansion of outer and inner nitrided layers 14, 15 inorder to control or restrict deformation or warpage of ring body 1.

Assuming that Δ F_(o) and Δ F_(i) respectively indicate outer and innercircumferential forces generated in each small area ΔA_(o) and ΔA_(i) onan imaginary sectional line m-n in ring body 1, and L_(o) and L_(i)respectively represent distances from neutral stress axis N to centralpoints of small area ΔA_(o) and ΔA_(i), if total or integrated momentsof force exerted on small areas ΔA_(o) and ΔA_(i) are substantiallyequal to each other, it can be appreciated that no deformation wouldoccur in ring body 1, as shown by the following formula (1):

Σ(ΔF _(o) ×L _(o))=Σ(ΔF _(i) ×L _(i))  (1)

In this case, distances L_(o) and L_(i) from neutral stress axis N tocentral points of small area ΔA_(o) and ΔA_(i) are not equal(L_(o)≠L_(i)) to each other. Suppose k=ΣL_(o)/ΣL_(i), equation (1) canbe developed to equation (2):

kΣΔF_(o)=ΣΔF_(i)  (2)

As forces resulted from expansion of nitrided layers are in proportionto areas of nitrided layers, if outer and inner nitrided small areas areshown by ΔA_(o) and ΔA_(i), equation (2) can be developed to equation(3). Also, assuming that A_(o) and A_(i) represent total outer and innernitrided areas respectively, equation (3) can be rewritten as equation(4). Moreover, supposing Y_(o) and Y_(i) indicate whole surface (edge)lengths of the outer and inner nitrided areas in cross section of ringbody 1 with the nitrided layer of constant thickness t, we have:A_(o)=Y_(o)×t and A_(i)=Y_(i)×t. If these equations are substituted forequation (4), we have equation (5).

kΣΔA_(o)=ΔΣA_(i)  (3)

kA_(o)=A_(i)  (4)

kY_(o)=Y_(i)  (5)

Accordingly, these equations show that ring body 1 should be formed witha constant ratio of outer surface (edge) length to inner surface (edge)length in section areas in order to control or suppress deformation dueto nitridation.

EXAMPLE

To test oil control rings according to the present invention, ringbodies of different cross section of nine (9) kinds were prepared andlapped with lapping agent untill lapped profile was formed on wholeouter circumferential surfaces of each ring body, measuring lapping timetherefor and checking status of lapped nitrided layers. Test results isshown in the following table wherein symbols show the following:

B(mm): Width of ring body; t (mm): Thickness of ring body; l₀ (mm): Land(Rail) width; l (mm): Length between rails; t₀ (mm): Groove depth; θ(degrees): Rail angle; τ: Circular ratio of an outer surface (edge)length Y_(o) to an inner surface (edge) length Y_(i) in cross section ofring body 1; T (seconds): Lapping time; A: Whole nitrided layers that donot survive lapping; P: Whole nitrided layers that survive lapping.

TABLE Test Result Sample No. B t l₀ l t₀ θ τ T A/P 1 1.5 1.5 0.18 1.060.7 9 1.01 150 A 2 1.5 1.5 0.18 1.06 0.65 9 1.09 103 P 3 1.5 1.5 0.181.06 0.60 9 1.18 80 P 4 2.0 2.0 0.18 1.41 0.75 10 1.20 80 P 5 2.0 2.00.18 1.41 0.65 10 1.32 103 P 6 1.5 1.5 0.18 1.06 0.5 6 1.37 200 A 7 3.02.0 0.18 1.70 0.9 10 1.16 85 P 8 2.5 2.0 0.20 1.50 0.82 10 1.24 85 P 94.0 3.0 0.30 2.90 1.5 20 1.08 104 P

Ring bodies of Samples Nos. 1 to 8 were formed into M-shape shown inFIG. 2 with annular concave groove 4, and ring body of Sample No. 9 wasformed into I-shape shown in FIG. 3 with tapered groove 4. The testresult reveals that Sample Nos. 1 and 6 do not have whole nitridedlayers because they were removed by lapping, and also that Sample Nos. 2to 5 and 7 to 9 bear nitrided layers left after lapping was carried outto generate lapped profile throughout the whole outer circumferentialsurface of ring body. FIG. 4 is a graph indicating the result of thetable, clearly demonstrating a fact that the ratio τ of an outer surface(edge) length Y_(o) to an inner surface (edge) length Y_(i) in the crosssection of the ring body 1 should be in a range between 1.08 and 1.32,more preferably between 1.0 to 1.3 with the following advantages:

[1] Dynamic balance can be maintained between forces produced in outerand inner nitrided layers 14, 15 in ring body 1 after nitridation.

[2] Ring body produces reduced or less strain, keeping preciseperipheral circularity after nitridation.

[3] Lapped profile of nitrided layer can survive lapping on outersurface of ring body 1 due to less distortion of ring body 1.

[4] Lapping time can be shortened for outer and inner nitrided layers14, 15.

[5] Lapping is facilitated.

[6] Yield of oil rings can be improved.

What is claimed is:
 1. A two piece oil control ring comprising a ringbody formed into a generally M- or I-shaped section with an annulargroove on an inside surface of the ring body; and a coiled expanderattached in the annular groove of the ring body for resiliently urgingthe ring body toward an inner wall of a cylinder, said ring bodycomprising an upper rail, a lower rail and a web that connects saidupper and lower rails, said web being provided with a plurality ofopenings for allowing lubricating oil to pass therethrough, said ringbody being made of stainless steel that contains chromium of 3.0 to 13.0weight % with the outer and inner circumferential surfaces covered withnitrided layers, and a circular ratio of an outer surface length to aninner surface length in the cross section of the ring body being in arange between 1.08 and 1.32.
 2. A two piece oil control ring as definedin claim 1, wherein said circular ratio of the outer surface length toinner surface length being in a range between 1.1 to 1.3.
 3. A two pieceoil control ring as defined in claim 1 or 2, wherein said ring body hasits width of 1.2 to 2.0 mm, thickness of 1.2 to 2.0 mm, land width of0.15 to 0.22 mm and annular groove depth of 0.5 to 0.75 mm.